IBM 5150 PC Technical Reference (6025005, August, 1981) (PDF)

Interrupts
One interrupt line is provided to the system. This interrupt is IRQ4 and
will be positive active. To allow the communications card to send
interrupts to the system, Bit 3 of the Modem Control Register must be
set = 0 (low). At this point, any interrupts allowed by the Interrupt
Enable Register will cause an interrupt. ,...-.......
The data format will be as follows:
TRANSMITTER OUTPUT AND RECEIVER INPUT
00 01 02 03 04 05 06 07
Transmit
Oata Marking
Data Bit 0 is the ftrst bit to be transmitted or received. The adapter
automatically inserts the start bit, the correct parity bit if programmed
to do so, and the stop bit ( 1, 1-1/2 or 2 depending on the comm.and in
the Line Control Register).
Interface Description
The communications adapter provides an EIA RS-232-C like interface.
One 25 pin "D" shell, male type connector is provided to attach
various peripheral devices. In addition, a current loop interface is also
located in this same connector. Ajumper block is provided to manually
select either the voltage interface, or the current loop interface.
The current loop interface is provided to attach certain printers
provided by IBM Corporation that use this particular type ofinterface.
Pin 18 + receive current loop data (20Ma)
Pin 25 - receive current loop return (20Ma)
Pin 9 + transmit current loop return (20Ma)
Pin 11 - transmit current loop data (20Ma)
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